Term usually applied to the anthropoid apes (primates) that are the closest relatives of humans. There are three great apes - chimpanzee, gorilla and orang-utan - and one lesser, the gibbon. An ape differs from a monkey in being larger, having no visible tail, and in possessing a more complex brain. Two monkeys are also called 'apes' - the Barbary ape of N Africa and Gibraltar, and the black ape of Celebes.

any primate of the superfamily Hominoidea, which includes humans; this article, however, focuses on the nonhuman apes. The small apes, the gibbons and the siamang, and the orangutans, which belong to the great apes, are found in SE Asia. The other great apes, the gorillas and the chimpanzee and closely related bonobo are found in Africa. The term ape was formerly and incorrectly applied to certain tailless monkeys. Ape and anthropoid ape are now used synonymously, although the common names of certain monkeys still contain the word ape; for example, the N African macaque is called the Barbary ape.

True apes vary in size from the 3–ft (90–cm), 15–lb (6.8–kg) gibbon to the 6–ft (1.8–m), 450–lb (200–kg) gorilla. All apes are forest dwellers and most spend at least some of the time in trees. Except for adult gorillas, they can run along branches on all fours; they are also able to move about by brachiation, or arm-over-arm swinging. Gibbons (including siamangs) are particularly adept at this type of locomotion; the heavier orangutans prefer to grasp a neighboring tree and pull itself across to it. Gorillas and chimpanzees are the most terrestrial of the apes, normally traveling on all fours by leaning on the knuckles of their forelimbs with the fingers of their hands curled under (knuckle-walking); orangutans ball their fingers into fists during the short periods they walk. Most apes are able to walk on two feet for short distances.

The skeleton of an ape is quite similar to that of a human in the structure of the chest and shoulders. Apes have broad, flat chests and arms capable of reaching up and backward from the shoulder; this construction is associated with brachiation. The pelvis, on the other hand, is more like that of a monkey, designed for walking on all fours, hence the use of knuckle-walking for ground locomotion. The arms of an ape are longer than the legs. The hands are similar to human hands, but with fingers and thumb of more equal length; the feet are handlike, grasping structures. Apes have neither tails nor the cheek pouches found in Old World monkeys; gibbons are the only apes that have the buttock callosities found in Old World monkeys. Like other anthropoid primates, the eyes are highly developed, with stereoscopic color vision. The brains of great apes are different from Old World monkeys and some structures are reminiscent of the uniquely elaborate features of the human cortex, rendering these primates capable of fairly advanced reasoning. Chimpanzees, bonobos, orangutans, and gorillas have been observed using objects as tools in the wild.

Estimates of the amount of identical genetic material (DNA) in chimpanzees and humans range from 94.6% to 99.4%. This marked similarity, and additional evidence, have led primatologists to suggest that the taxonomy of the apes should include two families: Hylobatidae (gibbons and siamangs) and Hominidae (orangutans, gorillas, and chimpanzees and humans). Apes are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Primates.